A conventional digital oscilloscope takes discrete periodic samples of multiple analog input signals and processes the samples to render on a display screen light images of the input signals. FIG. 1 is a schematic representation of a prior art method by which one type of digital oscilloscope acquires multiple similar waveforms 10a-10c of a continuous-wave sinusoidal analog signal 12. Waveforms 10a-10c are formed by multiple discrete voltage samples acquired at similar times during successive cycles of analog signal 12. Corresponding voltage samples are taken at similar times during each of the waveform portions and are identified by identical reference numerals and lower case suffixes, the latter of which indicates the particular waveforms. For example, samples 14a, 14b, and 14c and samples 34a, 34b, and 34c represent two different groups of three corresponding samples. The formation of a digital signal is described below by way of example mainly with reference to waveform 10a.
Waveform 10a is represented by a data record that includes digital words representing multiple discrete voltage samples 14a-34a acquired during an acquisition period 36a. Samples l4a-34a represent the measured voltage amplitude values of analog signal 12 at periodic time intervals. The measured amplitude values typically form the vertical component of a display waveform rendered on the display screen of the digital oscilloscope. The horizontal component of the display waveform is typically formed by a time base sweep representing an acquisition period.
Acquisition period 36a begins at the end of a trigger system reset period 38a in response to a triggering event 42a. Triggering event 42a occurs after reset period 38a when analog signal 12 crosses a preselected trigger voltage level 44 with a slope of a predetermined polarity.
Analog signal 12 typically includes an a periodic noise component (not shown) characterized as a random variation in the voltage amplitude (i.e., the vertical component) of analog signal 12. Digital storage oscilloscopes of certain types correlate corresponding samples of waveforms 10a-10c and compute for each group of them an average voltage amplitude, thereby to form a display waveform having a voltage amplitude representing the average voltage amplitudes computed for different times in a cycle of analog signal 12. The averaging of corresponding samples can form a display waveform having a better vertical resolution (i.e., signal-to-noise ratio) than the vertical resolution of each of the acquired waveforms 10a-10c. The reason is that the averaging of relatively large numbers of corresponding samples tends to "average out" the noise component of analog signal 12.
In an electrically noisy environment, however, the voltage amplitude of the noise component of analog signal 12 can be comparable to that of its periodic signal component. Such a noise component could cause the voltage amplitude of analog signal 12 to cross trigger voltage level 44 at random intervals, thereby erroneously triggering any of the acquisition periods 36a-36c. Such an erroneous triggering results in a time misalignment or offset of samples otherwise corresponding to one another. The correlation of corresponding samples during averaging would, therefore, be improperly performed and, as a consequence, would result in the formation of different display waveforms that do not accurately represent analog signal 12. Moreover, the different display waveforms could cause the image representing analog signal 12 to appear unstable, a condition that is sometimes called "trigger jitter."